Passive position determining system



June 7, 1960 T. B. BISSETT ET AL PASSIVE POSITION DETERMINING SYSTEMFiled Feb. l5, 1957 @arm/a for Genf:

United States Patent Passiva rosrrroN nnrnnsmsnNG SYSTEM Thomas B.Bissett, Redondo Beach, Wiley V. Conover, Tarzana, and Albert S. Fulton,Manhattan Beach, Calif., assignors, by mesne assignments, to IThompsonWooldridge Inc., Cleveland, Ohio, a corporation o o Filed Feb. 1s, 1957,ser. No. 640,412

s claims. (ci. 343-112) This invention relates to a position determiningsystem, and more particularly to such a system as will determine thelocation of an energy radiating source by means of passive listeningdevices with respect to which the radiating source bears relativemotion.

The most effective existing technique for determining the position ofenergy radiating sources utilize two or more receiving stations whichmust determine the directions from which the energy originates.Direction finding static-ns that are capable of providing the necessaryaccuracy require large and costly antenna installations and are, ingeneral, slow in operation, since the antenna must be pointed in theproper direction to receive the radiation.

With the tremendous increase in the amount of air traic has come therealization that the existing techniques are inadequate for providing anaccurate 'and continuous means of tracking moving aircraft. The increasein the number of air collisions by the aircraft bearing relatively highvelocities with respect to one another indicates that an improvedtracking system is necessary to allow ethcient and safe navigation on orolf the recognized airways.

The position determining system herein disclosed obviates one of themajor disadvantages previously noted in connection with presentlyexisting systems in that it determines the position of an energy sourcein a matter of seconds. Further, only passive receiving devices areneeded, thereby simplifying and reducing the cost of ground stations, inaddition to increasing the speed of system operation. Moreover, nospecial equipment need` be installed upon the craft being detected,since any craft emitting modulated energy can be located accurately.Previous position determining systems utilizing radar apparatus hadgreat ditliculty in detecting small private aircraft because of theirsize; however, the present system can accurately locate such aircraftprovided only that they emit some form of modulated energy such as forexample a voice modulated radio carrier wave.

The present invention is based upon an improvement in what may be termeda hyperbolic line of position system of direction and/or positiondetermination. Many systems of this general type have been proposed inthe prior'art, as for example shown in the British Patent 7,172 of 1915,and the patent to J. B. Morrill, 1,406,996, issued February 21, 1922. Inthe prior art, position determining systems, based upon the use ofhyperbolic lines of position, have taken form in arrangements whereinthe operator of a craft may determine his position by comparing therelative time delay between versions of specially constituted radiosignals received from av plurality of fixed ground based -broadcaststations. The loran ship navigations system is typical of this type ofsystem. Alternatively, there has been suggested a system which issubstantially the reciprocal of the loran system in which a plurality ofspecial radio transmitters are borne by a craft to permit a like`Patented June 7, 1960 pige There has also `been suggested a system of`bearing determination of this latter reciprocal type in which two ormore directional receiving stations monitor energy emanating from acraft with each orienting the axes or lines of antennae directivities oftwo antennae, so that the versions of received energy are brought intotime coincidence. The orientations of the antenna then define thegeneral direction of the craft. These prior art techniques, however, donot offer as suitable a solution to the problem of air traic control asthat which the present invention provides. This is because these priorart systems either require special radio' transmitting equipment to beborne by the aircraft or they fail to take into account the effect ofthe relative velocities of aircraft with respect to the receivingstations. This velocity consideration produces what is sometimes termeda Doppler shift in the frequency of the wave energy received from theaircraft. The magnitude of this frequency shift is potentially differentfor each receiving station due to the fact that the component ofaircraft velocity in the direction of each station, may be different.

The present invention, therefore, takes advantage of the fact that ingeneral, energy intentionally radiated from 1 an aircraft, such as radiocarrier transmissions, are purposely modulated with signal informationof a type which is aperiodic and multifrequency in character.rIherefore, although the radio carrier wave, per se, may sufferlsociated with the carrier frequency. Advantage is further taken of thefact that whereas interference or interferometer type null detectionschemes have been useful in the prior art for determining thetime-difference between two versions of a signal comprised substantiallyof sinusoidal components bearing a predetermined frequency versusamplitude distribution, such null techniques are not capable ofproviding sharp indicia of time agreement between two versions of anaperiodic multifrequency signal. This is especially true underconditions where this latter type constituted signal is contaminated bysubstantial noise.

The present invention, therefore, embraces the use of,

a signal correlation system permitting two versions of an aperiodicYmultifrequency signal to be compared in a mannercausing the effects ofnon-coherent componentsV in the two versions to cancel each other over apredetermined time period which may be termed a smoothing or averagingperiod. This type of signal correlation process, when used to determinethe time relationship between the coherent components of two versions ofanintelligence modulated radio carrier, permits Doppler shift errors andrandom non-coherent noise eiects to be minimized, thereby yielding datafrom which hyperbolic line of position fixes may be quickly obtained inaircraft controlV work.

IIt is, therefore, one object of this invention to provide a low cost,passive position determining system `for determining the location of amodulated energy radiating source bearing relative motion with respectto an observation means. Y I V It is another object of this invention toprovide a passive position determining system that is capable ofcontinuous-Y ly and accurately tracking moving craft. I

It is a further object of this invention to p'rovide a passive positiondetermining system in which the' craft to be 2,940,076 Y l e Y., i

located need only have a minimum of radio equipment to bev positivelyidentified and tracked.

'It is a still further object of this invention to provide a passiveposition determining system that can be fabricated from presentlyavailable components, and that willV tially different velocities. Meansresponsive to the modul-Y ation borne by the radio carrier wave Varethen provided forso correlating the two ldifferent versions of thereceived radio signals provided by the two stations, `that effects ofnon-coherent components in the, two signalV versions are caused toaverage or cancel out over a pre# determined smoothing or averagingtirneinterval. By

imposing a controllable value of relative time` delaybetween the twosignals, during this correlationprocess, a sharp Vaccurate indication ofthe time displacement between the two versions of the carrier asreceived at the two receiving locations may be determined. Through theuse of this type of correlation means, the degree of accuracy of thisindication is` highy compared to the'percentage' of the potentialDoppler frequency 'shift distoi'-, tion imposed on the radio carrierwave assignable to any,Y

difference in the relative velocities of the source with respect to thetwo receiving stations. I

In further accordance with'the present invention, at least threereceiving stations areprovided, each physically separated from the otherby known givenl distances `and directions forY monitoring a given areaof geographical terrain over which it is expected that a radiortransmitter t earing craft may pass. Means are provided at eachreceiving station, for receiving incoming radio carrier waves;

modulated by aperiodic multifrequency information, such, as voiceinformation signals. VThese stations, forming three pairs of stations,are further adapted to simultanel ousiy receive andy respond to the sameradio carrier wave,

by the stations to permit the position of the transmitter at the time ofits transmission to be ascertained.

The features of the inventionthat are believed to be novel andpatentable are pointed out in the claims which form a part of thisspecification; for a better understanding of the invention, reference ismade in the following description to the accompanying drawing, whereinlike parts are indicated by like reference numerals, in which:

Figure lY is a block diagram illustrating the major components of thepresent passive position determining system; Y

Figure 2 is a sketch illustrating the geometry involved Y in making thecalculations involved by the present system; and Y Figure 3 is a blockdiagram, a portion of this gure being shown in perspective, of thecorrelator and comdetermining services of the invention.

' values.

puter apparatus that performs the necessary position Referring now toFigure l, there are shown three receiving stations respectively labeledAWB, and C. Each ofV these stations receives modulated energy radiatedfrom an energy sourceinot shown) whose position itis de,- sired todetermine. The stations are preferably lfixed in position and separatedfrom one another by known disv tances. The distances between thestations must be aocurately known, although they need not have anyspecial' piece of apparatusV labeled Correlator and Computer Center. Atthis correlator and computer'center the differences between the pathstaken by energy from the source to, each pair ofv stations is measuredand utilized to determine the position of the source, as will beexplained hereafter.

Referring now to Figure 2, there will be seen a pair of axes labeled xandy on which are shown a pair of symmetricalhyperbolas 1 and 2.Hyperbole. 1 has its centerA at point minus c on the :cV axis andhyperbola 2 has its center at plus, con the x axis. The hyperbolacenters plus c and minus c, respectively, represent the positions of`two of the stations shown in Figure l. The distance between any point onhyperbola 2 and the center of hyperbola 1 is labeled R1, and thedistance between thatv so that a carrier wave, for' exampleemanatingfrom an aircraft voiceY type radio transmitter, is Areceived at alllrelative time between the Vversions of each pairduring the correlationprocess, at least two time diiference signals are developed which aresharp indicia of the rdilerences in the times of the arrival of thecarrier" wave at the members of different pairs of stations. VThesetimediiference signals correspond tol distance-dilerence. informationdepictng the diiferencc inthe lengths of the paths traversed by theradio carrier Ywave energy 'in its propagation from the transmitter tothe members of different pairs of the receiving stations. Thesetime-difference signals, along with data signals representing the knownseparation and relative positionsV of .thestationsgy may then be appliedto a computer so programmed as to compute the intersectionof at leasttwo hyperbola, each hyperbola being one of a different set ofhyperbolas, with each different set being in turn based upon the posi-Ytions of a diierent pair of stations as foci. The p'oint of intersectionof these hyperbolas maybe expressed in' a separate system of coordinatesserving the area monitored These signal versions are then proc-V samepoint and the center of hyperbole 2 islabeledtRg. Since a hyperbole. isdefined at., the locus of a point such that the difference of, itsdistances from two Vfixed points is constant, it will be apparent thatRl-RZ equals acon# stant. This constant difference lis equal to 2a wherea is thentersection of the hyperbolas with the xV axis, this.

equation ,beingderived from the naturelof ahyperbola.

Now, the equation` foraV hyperbola can be stated as' follows 1 j i (l)If the distance between two stations or foci of the pair of hyperbolasis considered to be d, then:

v l d=2c Y f (2) Further, f romv the foregoing it will be apparentAthat: l

' t ZaFRif-Ra Y (-3-) .Squaring both sides of v(3) we ind: Y

az: (R1-l; Re? K y ,(4).

Moreover, from (2) we find:

Cz Y Y (5) The aforesaid stations Vare connected by means, of linkslabeled Inter-Station Communications to a singlek Examining Equation 6it will be'noted that aside from the X and Y terms therein the remainingquantities are either known constants or can be measured. Thus d is theknowndistance between the pair of stations for which the hyperbolas arebeing constructed, and Rl-Rz is determined at the correlator andcomputer center of Figure 1. Therefore, once R1-R2 is measured anequation for X and ,Y only can be obtained, and when two such equationsare obtained from two pairs of stations the inten' section point of thehyperbolas will determine the locationof `the energy source whosedierences in distance from the stations were measured in order todetermine the hyperbolas. This general treatment is more fullyconsidered in the above referenced British P atent 7,172 of 1915.

A In order to obtain Rl-Rg the two signals received by two stations fromthe same source are matched or correlated and the time delay required toachieve this match will be directly proportional to R1-R2. To measurethis time delay, the signals according to the present invention must bemodulated, so that they may be matched with a high degree of accuracy,so that the difference between their starting times may be ascertained,and to insure the fact that they are both being radiated from the sameobject. Furthermore, as described above, it is anticipated that thesource may comprise a radio transmitter borne by a movingaircraft. Sincethe receiving stations are spaced apart from one another, the value ofrelative velocity of aircraft motion with respect to each station may bediierent. Under such conditions, due to the well known Doppler effect,the etfective frequency of a given radio carrier transmittedlfrom anaircraft, as sensed by each station, may be different. vFor example,assume a jet. aircraft is traveling along a straight line pathconnecting stations C and B in Figure l, and as first monitored, theposition of the aircraft is to the right of station C. but moving in adirection toward station C and station B. Also assume that the groundvelocity of the aircraft is 500 miles per hour and that all receivingstations are displaced from one another by 100 miles. If an unmodulatedradio carrier, broadcast from the aircraft, was in the order of 100 mc.,the signal received by stations C and B would be as much as 100 cycleshigher than the signal received by station A. Furthermore, as theaircraft moves along its path, the difference in the frequencies of thereceived signals will vary or change due to changes in the relativevelocity of the craft with respect to the receiving stations.

If prior art modulation signal matching techniques, such as described inUnited States Patent 2,166,991 to Guanella were employed in an attemptto correlate received versions of the unmodulated carrier, little or notime' difference information could be obtained since such product ormodulation matching would yield substantially no average outputindication. That is, an effort to nd a correlation or agreement betweentwo signals of varyingly different frequencies, over reasonable valuesof smoothing time, would result in information indicating that nocorrelation or agreement exists. In other words, due to Dopplerfrequency shift effects, inherent in the receiver-transmitter positionrelationship contemplated in an air tratc control system embodying thepresent invention, there may be substantially no coherency between thereceived versions of an unmodulated carrier. This consideration is ofparticular importance in practical cases where generally encounteredsignal to noise ratios in the order of unity may exist at the receivinglocations.

The present invention, however, overcomes this dithculty` -by providingtime-difference measuring means responsive to modulation borne by thecarrier and of a type which averages out the effects of non-coherentcomponents (such as noise and Doppler shift components) appearing in anytwo versions of the received signal so that correlation of those aspectsof the received signals representing the modulation intelligence may beeffected.

More specically, it is Well known in the art, that if the intelligenceborne by the modulated carrier is comprised of signal frequencies themaximum value of which is a small percentage of the radio carrierfrequency, the modulation intelligence borne by the carrier will beexpressed in the form of radio frequency side bands closely associatedin frequency with that of the radio carrier itself. The difference inthe frequency of the various side bands and that of the carrierrepresent the frequency make up of the signal intelligence borne by themodulated carrier. It is this type of situation that the presentinvention contemplates, as for example a radio carrier of mc. modulatedby voice signal intelligence for example covering the frequency range of200 to 3,000 cycles per second. Under these conditions Doppler shift `ofthe radio carrier and side bands due to relative velocity of `theaircraft will be expressed in equal percentage frequency of both thecarrier and side bands. The resulting shift in the frequency of themodulation intelligence expressed by the difference in the frequenciesof the side bands and theV carrier will, however, be 100,000,000 timesless. Thus by determining the time difference between twoversions of thesame modulated radio carrier through the use of the above type ofcorrelation process, which process is responsive to the modulation borneby the carrier, high accuracy in time ditference measurement may beobtained. Furthermore, since the voice modulation of a carrier isaperiodic and multifrequency in character, correlation processes of thetype anticipated by the present invention provide quick and positiveposition determination vby avoiding ambiguities which might otherwisearise through an attempt to correlate or otherwise time match, twounmodulated radio signals of substantially the same frequenciesemanating from two aircraft closely positioned with respect to oneanother while in flight.

Referring now to Figure 3 there is shown a typical correlator andcomputer center that could be used to accomplish the objects ofthisinvention. As will be noted, the three stationsA, B, and C are eachshown as having a respective device for receiving modulated energy froma radiating source, such devices being depicted as antennas and beinglabeled 3. Each antenna 3 is respectively connected to a respectivereceiver 4 in order that the energy received by the antenna may beamplied and transmitted through communication links -6 to the correlatorand computer center. These communication links may be cables laidbetween the stations and the correlator and computer center, or thestations may transmit their received signals to the center by means ofradio or radar communication systems. The antennas, receivers, andcommunication links are all well known in the art and so no furtherdescription is deemed to be necessary here. Suflice it to say that theinvention is not limited to radio energy but will operate effectivelywith any other type of modulated energy emitted by the source. It will.be obvious that the antennas and receivers will vary in type with theenergy being detected and amplified.

Let us now examine the correlator center portion 5 of the correlator andcomputer center. The modulated signals from a pair of communicationlinks 6 are fed through a pair of magnetic recording heads 7 andmagnetically recorded upon a recorder 8 as a pair of signal tracks 9.recorder 8 being rotated by a motor 10. A pair of magnetic reading heads11 are respectively associated with the respective tracks 7 and theyproceed to pick off the signals from these tracks. One of the heads 11is physically displaceable from the other so that a time delay will beintroduced between the signals so picked olf. A timing reference device12, through a pair of coils 13, places suitable magnetic timing marksupon tracks 9 so that the signals impressed by heads 7 may be timereferenced.

The signals and timing reference marks from heads 1.1 are both fed intoa correlator 14, and pair of coils 16 also feed the timing marks into atime distance converter 17. Correlator 14 multiplies together the twosignals fed into spigot-.hing tinie- Movable head ll--is displaceduntilithis while the present.. invention not; necessarily limited. to.

` suchv a eonguration, the-preferred. en nbodirnent` of; th .inr-

averaged productbecornes. a maximum, the-event osfithieY Y maximumindicating that the coherent aspects ofthe modulation borne by therecorded signals are then in substan-v tial time coincidence as they--reach the correlator 14.

'lhe delay introduced by movable head. 11. will then be found.. to. bedirectly proportional to the diiierence hetween. the two paths taken byenergy from the source to stationsv A and B. Time distance converter 17isA merely acounter which measures the difference between the. urn--ing.: reference marks. between heads ll when the outputv trein.correlator 14 isa this. difference measuring-the-foregoing; delay- Flheoutput iro-rn timev distance converter-17. will then. be directlyproportional to R1. 2.,' Since a.; sharp. rnaainiutn. will onlyV beobtained when eolierent'niodulatiou espec-ts identical signals arematched;

` sinnilary input. is. fed into the. computer from'V other cor.-

relatorl center 5. which performs the same'operations as; thefpregoingupon signals receivedr bystations YB and C.. Into the computer is alsofed a signal 2c: for each pair oi stations and; corresponding tothedistance between each" f; stations. As was notedv above in connect-ion`with.

thev QQDlltlter' enables thecomputersimultaneously.I to def.

so de :ji-ned in, order to. find the exact values of X and Ykcorresponding to the position of thev radiating source of energy;Computer .ltranslatesthe Xand Y -ofeachpair of stations onto a commonpair of aires. As. will also be noted; in connection with this figure,athird inputto the computer could be from another correlator center 5uti. lining inputs from stationsA A andC,'and its associated, signal 2qwould be that corresponding to this. third pairof stat-ions, A thirdhyperbole. would thereby.I bedened andl itsintersection Withthe othertwo would serve to deine more, accurately the position of the radiatingsource.

Both the correlator center and the. computer have beenshown; iny blockdiagram form becausev they are well lenga/nin the. art. With respect tothe timing, recording, and correlation portionsof the equipment, onetype that is. suit-able; for use in the. present system is depicted andY described in a report Vpublished by the Omce of Naval Rgsearoh,Technical ReportNo. 144., by. R; A. Johnson of; Cruft- Laboratory,Harvard University, Cambridge, Massachusetts, March 1952, and entitledAn Analog Computer for Correlation .Functions in Communication Systems,A suitable computer wouldV be the Digitac computer lmanufactured by theHughes Aircraft Come.

pany, and other suitable computers for performing the relativelysimplecomputations required. by this system arealso commercially available.

It should be. understood that the spacing and location of the. stationsand correlator and computer center are notcritioal nor is the presentinvention limited to any par-ticulai; type of; computer, receiver, orantenna. The correlator-perse, may also take ya variety-of formsprovided, as pointed outabove, that the correlation is carried out inresponseto the modulation bearing aspects of the received signalversions. It shouldvalso be clear that although only llllleef stations Yare. shown and described, this is merely becausel threeY stations mosteconomically' can form eithertwofor three pairs of stations. It iscertainly Within the scope of the. invention Vto provide four stationsfor two pairs or six stations for three pairs, Wheret-his is deemed tobedesirable.

Further, thehyperbolas derived from two pairs of stationsmaytheoretically intersect in more than one point, or; else.theirintersection Vangle may be sov small asto make-v thirintersection.point diicult torefad. Consequently,

ouation, 6 the foregoing information when applied to inet-heintersectionpoint of thel various; hyperbol-as;

- where only two hyperbolas: are used, the obviously impossible pointsof ambiguousintersection. can. be suppressedY by the computer so. as toavoid the ultimate development. o-f-ifalsey position information.'Moreover, itj will be obvious, that onev communications link will be.saved when the correlator and computer cen-v ter is disposed at one ofthe stations, andv that the cornV putations and equipment involved maybe simpliied by arrangingy the three stations.y to form an. equilateraltri:y angle, thereby causing 2c to be the saine constant for eachpair ofstations. Also, it will be obvious that some, oi the apparatus inthecorrelator center illustratedinFigure 3 could. be. common tothe othercorrelator'centersI For. example, the timing; reference, motor, V.tiinedistance. con; verterfaud.- correlator could be common to all. thecorre; later centers.- Perhaps a. multichanneltrecorder. could also beused f or all of the correlator centers. .Y

WhileY there h as'been described what is at present conf.. sidered thepreferred embodiment of the invention, it will be obvious tov-those.skilled inthe art that various changes: and modicationslrnay bemadetherein without departing. freinY the.- invention and; it. is aimed. inthe. appended claims to cover al1y suchchanges andmodications. as fallwithin; the true spiritgand scope. ofthe` invention. Y

What Lclaini as new andf desire tosecure by Letters.. Patent ofA theUnited States is.: 1

1.-A In. a'target locating system. useful in determiningthe positionof.4 a. niovingicratt f by intersecting.. hyperbolic lines', ofposition,j and from. which. craft conditionally ernanated` a periodiccarrier. wave of; nominal frequency,- value. said wave.-beingl modulatedby aperiodic rnulti.-` frequency Ymod.ulation. components, ther nominalf res quency value o t said 'carrier wave being relatively high. with.respect to the highest. frequency represented. said informationcomponent, the combination oi.:- at least a irst receiving. station anda. secondl receivingY station displaced, froml one another by arelatively large known distancersuch that the, respective magnitudes ofvelocity,y which a givenl moving craft Abears relative to said sta--tions, are conditionally different from one another by a..

substantial amount,vr the-:difference in fsaid magnitudes, oi Y velocityinI turn causing the apparent nominal frequency.

' of. a given` carrier wave arriving at one station tov subatirstelectrical signal including a.. periodic carrier; come pompent andaperiodic rnultifreriuencyv modulation corn-y ponents;` secondtransducing means;A included at` said secr. 0nd. receiving station fortransducingr said saine given carrier Wave, as. received at saidl seconds tatiom into a.

second' electrical signal including'a periodic carrier. com; ponent `andaperiodic multifrequency. modulation. corniponents; means responsive tosaid lirsty and said second electrical. signals for: developing, an.output signal, repre-- l senting substantiallyonly the timeaveragedVmutual prod,- uct: of. said modulation components. included.y in.. saidand said second electrical signals;V and rneans interposed between;-atrleast-one. oliy s .d transducing means andsaid last named means forimposingeontrollably variablevalues of relative; delay between the1n.edulf'ition-.corn.-n

ponents inc ludedin said; first. and; Secondi electrical signals for.controllingthe. value of: said. output; signal..

2.-. In a, target locating systein useful in. dete the position of a.moving crate by intersecting hyperboliclines of-y position, andfrorn.which Vcraft-is conditionally emanated Ya periodic car-.rior wave. of.nominal frequency value, said wave beingmodulated by.- apar-iodioJnini-iti-A gfrequency modulation components, .thenominalffrequeucy valueof said carrier wave being relatively high with respect to the highestfrequency represented in said infomation component, the combination of:at least a first and a second receiving station displaced from oneanother by a relatively large known distance such that the respectivemagnitudes of velocity, which a given moving craft bears relative tosaid stations, are conditionally diierent from one another by asubstantial amount, the dilerence in said magnitudes of velocity in turncausing the apparent nominal frequency of a given carrier wave arrivingat one station to substantially diier from the apparent nominalfrequency of that same carrier wave during its arrival at the otherstation; first transducing means included at said first receivingstation for transducing a given carrier wave of the type described, asreceived at said rst station, into a iirst electrical signal including aperiodic carrier component and aperiodic multifrequency modulationcomponents; second transducing means included at said second receivingstation for transducing said same given carrier wave, as received atsaid second station, into a second electrical signal including aperiodic carrier component and aperiodic multifrequency modulationcomponents; means responsive to said iirst and second electrical signalsfor developing third and fourth electrical signals, respectively,representing substantially only the modulation components included insaid irst and second electrical signals; means responsive `to said thirdand fourth electrical signals for developing an output signalrepresenting the time-averaged mutual product of the modulationcomponents included in said first and second electrical signals; andmeans interposed between at least one of said transducing means and saidlast named means for imposing controllably variable values of relativedelay between said third and fourth electrical signals, whereby therelative delay between said third and fourth electrical signals may becontrolled to produce a predetermined value of said output signal. l

3. In a target locating system useful in determining lated radio carriersignals at a given assigned signal frequency, the combination of: aiirst, a second and a third receiving means each positioned atrespectively dilerent and substantially displaced locations on theearths terrain for receiving modulated radio carrier signals atsubstantially said given assigned frequency and demodulating suchreceived signals to respectively produce a first, a

second and a third intelligence signal, each representing substantiallyonly the modulation components of modulated carrier signals received bya respectively diierent one of said receiving means; rst signaldeveloping means responsive to said iirst and second intelligencesignals for developing a iirst output signal representing thetimeaveraged mutual product of said rst and second intelligence signals;iirst delay means interposed between at least one of said first andsecond receiving means and said iirst signal developing means forimposing *controllably variable values of relative delay between saidfirst and second intelligence signals; second signal developing meansresponsive to said second and third intelligence signals for developinga second output signal representing the time-averaged mutual product ofsaid second and third intelligence signals; second delay meansinterposed between at least one of said second and third receiving meansand said second signal developing means for imposing controllablyvariable values of relative delay between said second and thirdintelligence signals; and means responsive to predetermined values ofsaid first and second output signals, taken in combination with thevalues of relative delay imposed by said first and second delay meanswhich produce said predetermined values of said irst and second outputsignals, for developing the positions of and distinguishing vbetweenindividual members of a plurality of craft bearing relative motion withrespect to the earths terrain, said craft conditionally andcontemporaneously transmitting intelligence moduposition datainformation depicting the position of a given craft relative to saidreceiving means.

References Cited in the file of this patent UNITED STATES PATENTS1,406,996 Morrill Feb. 21, 1922 2,166,991 Guanella July 25, 19392,587,467 Hawkins Feb. 26, 1952 FOREIGN PATENTS 675,992 Great BritainJuly 23, 1952

